This invention relates to motor vehicles having an internal combustion engine with an external exhaust gas recirculation system in the form of an exhaust return line connecting an exhaust line to an intake duct and having a passenger compartment with a heater with a heating circuit containing a heat transport fluid.
Motor vehicles are commonly powered by internal combustion engines. Conventional internal combustion engines are piston engines having an intermittent mode of fuel combustion which results in non-optimal combustion processes. Consequently, in addition to harmless combustion products such as water and carbon dioxide, substantial amounts of atmosphere pollutants are formed. Among the most significant pollutants generated by internal combustion engines in the exhaust gas are oxides of nitrogen (NO.sub.x). The formation of nitrogen oxides is critically dependent on the peak combustion temperature. To reduce the peak combustion temperature, some internal combustion engines have been equipped with exhaust gas recirculation arrangements. In external exhaust gas recirculation, an exhaust gas return line connects the exhaust duct of the engine to the intake duct. A portion of the exhaust gases displaced from the combustion chamber during the exhaust stroke of the piston is recirculated through the exhaust return line to be supplied to the combustion chamber during the next intake stroke together with a fuel-air mixture, in the case of an Otto engine, or combustion air in the case of a Diesel engine.
The proportion of recirculated exhaust gas in the cylinder charge cannot be increased above a certain level because, if the fuel mixture is too lean, emissions of hydrocarbons will increase.
Another way to reduce pollutant emissions in internal combustion engines is to reduce the specific fuel consumption. Modern Diesel engines with direct injection may obtain such high efficiencies in this way that vehicle engines of average size often fail to supply enough heat to the engine coolant during operation in the partial load range to heat the passenger compartment of the vehicle sufficiently at low ambient temperatures.
Consequently, supplementary electric heating systems producing a heat output between about 500 W and 1 kW have been proposed for heating the engine coolant under unfavorable operating conditions, i.e., to permit heating of the passenger compartment when the engine is running at partial load and outside temperatures are low. The cost involved in providing such supplementary electric heating systems includes, in particular, larger electric generators. In addition to the increased manufacturing cost, the vehicle weight is undesirably increased, contradicting the primary objective of achieving lower fuel consumption.
Special heat exchangers located in an exhaust line have also been proposed, for example in German Patent No. 31 03 198, to provide additional coolant heating capability. Such heat exchangers do withdraw heat from the exhaust, but are installed at a location in the coolant line at which there is no positive effect on the combustion temperature and, therefore, they cannot produce any reduction of nitrogen oxide emissions.